Gravest Empirical Mode to be used by Inverted Echo Sounders in order to determine the zonal flows in the south Atlantic

Four Pressure-equipped Inverted Echo Sounders (PIES) were deployed at about 10°W, between 19 and 35°S, in the South Atlantic Gateway (SAGA), in order to determine the zonal flows in the South Atlantic. Those PIES will allow to observe the circulation of two water masses, the South Atlantic Central Water (SACW) and the North Atlantic Deep Water (NADW), that flow in opposite directions across the South Atlantic, between Cape town and Brazil, through the SAGA. The measurements from the PIES, together with historical hydrographic data, permit to estimate the profiles of temperature and salinity of the water column, and therefore the density. Using the thermal-wind equation, it is also possible to retrieve the geostrophic velocity from an array of PIES. In order to get those estimations of temperature and salinity, it is necessary to determine the Gravest Empirical Mode (GEM), a relationship between the acoustic travel time observed by the PIES and the historical observations of salinity and temperature in the study area. In this work, we will show the GEM estimated for the SAGA, calculated using historical hydrographic data from CTDs and Argo Float, as well as the estimations of the error in the geostrophic transport